1. Field of the Invention
This disclosure relates in general to magnetic storage systems, and more particularly to a method, apparatus and program storage device for dynamically adjusting the write current in each head to compensate for variation in disk drive and environmental parameters.
2. Description of Related Art
Disk drives are used as primary data storage devices in modern computer systems and networks. A typical disk drive comprises one or more rigid magnetizable storage disks, which are rotated by a spindle motor at a high speed. An array of read/write heads transfer data between tracks of the disks and a host computer. The heads are mounted to an actuator assembly that is positioned so as to place a particular head adjacent the desired track.
Information is written on each disk in a plurality of concentric tracks by a transducer assembly mounted on an actuator arm. Typically, the transducer assembly is suspended over the disk from the actuator arm in a slider assembly, which includes air bearing surfaces designed to interact with a thin layer of moving air generated by the rotation of the disks. Transducer assemblies are said to “fly” over the disk surface as the disk rotates. To access tracks on the disk, the actuator arm moves in an arc across the disk. The physical distance between the nominal centers of two adjacent tracks is referred to as the “track pitch”. The track pitch and linear track density define the storage capacity of the disk.
Each of the disks is coated with a magnetizable medium wherein the data is retained as a series of magnetic domains of selected orientation. The data are imparted to the data disk by a write element of the corresponding head. The data thus stored to the disk are subsequently detected by a read element of the head. Although a variety of head constructions have been utilized historically, magneto-resistive (MR) heads are typically used in present generation disk drives. The write element of a magnetic head uses a thin-film inductive coil arranged about a ferromagnetic core having a write gap. As write currents are passed through the coil, a magnetic write field is established emanating magnetic flux lines from the core and fringing across the write gap. The flux lines extend into the magnetizable medium to establish magnetization vectors in selected directions, or polarities, along the track on the data disk. Magnetic flux transitions are established at boundaries between adjacent magnetization vectors of opposite polarities.
To write a computer file to disk, the disk drive receives the file from the host computer in the form of input data that are buffered by an interface circuit. A write channel encodes and serializes the data to generate a data input stream that can be represented as a square-wave type signal of various lengths between rising and falling signal transitions.
A write driver circuit uses the data input stream to generate a write current which is applied to the write head, creating the magnetic write field that writes the encoded data to the magnetizable medium of the selected disk. The write current both reverses the polarity of the magnetic write field, creating the magnetic flux transitions, and sustains a given polarity between successive magnetic flux transitions.
A write head typically employs two ferromagnetic poles capable of carrying flux signals for the purpose of writing the magnetic impressions into the track of a magnetic disk or tape. The poles are fabricated on a slider with the pole tips located at the air bearing surface. Processing circuitry digitally energizes the write coil that induces flux signals into the poles. The flux signals bridge across the write gap at the air bearing surface so as to write the magnetic information into the track of the rotating disk. The thinner the thickness of the write gap layer, the greater the number of bits the write head can write into the track.
A write head is typically rated by its areal density that is a product of its linear bit density and its track width density. The linear bit density is the number of bits that can be written per linear inch along the track of the rotating magnetic disk and the track width density is the number of tracks that can be written per inch along a radius of the rotating magnetic disk. The linear bit density is quantified as bits per inch (BPI) and the track width density is quantified as tracks per inch (TPI). As discussed hereinabove, the linear bit density depends upon the thickness of the write gap layer. The track width density is directly dependent upon the width of the second pole tip at the ABS. Efforts over the years to increase the areal density of write heads have resulted in computer storage capacities increasing from kilobytes to megabytes to gigabytes.
There are many parameters critical to the ability of the write head to alter the magnetic information in the media. Some of the more salient factors are the flying height of the write head (FH), the magnetic parameters of the media (coercivity, magnetic remanence, thickness), the material and physical size of the poles, and the write current. In general, most of the critical parameters have associated tolerances, both as the components are received, and as the environment of the assembly varies. Among the salient parameters, the two most important from a writing standpoint are the flying height (FH), and the media coercivity, (HC). As the mechanical tolerances of the slider vary, the flight height can either increase or decrease. Similarly, temperature or air pressure can significantly change the flying height. In a like manner, external environment such as temperature can change the coercivity of the media.
It can be seen that there is a need for a method, apparatus and program storage device for dynamically adjusting the write current in each head to compensate for variation in disk drive and environmental parameters.